1. Field of the Invention
The present invention relates to a system and method for adjusting the image quality of a liquid crystal display by adjusting the potential on the common electrode in such a way as to reduce flicker (i.e., fluctuation of brightness) produced on the viewing screen of the liquid crystal display.
2. Description of the Related Art
FIG. 10 is a block diagram of the prior art driver circuit for a liquid crystal display. Shown in this figure are a liquid crystal panel 1 consisting of two glass substrates and a liquid crystal material sandwiched between the glass substrates, a signal-side driver IC 2 for driving the liquid crystal panel 1, a scanning-side driver IC 3 for driving the liquid crystal panel 1, and a control circuit 4 for supplying control signals to the signal-side driver IC 2 and to the scanning-side driver IC 3. The control circuit 4 also supplies a scanning signal 5 and a display signal 6. A large number of pixels for making up an image are arranged in rows and columns on the liquid crystal panel 1. FIG. 11 is an enlarged view of some pixels.
FIG. 11 is a diagram illustrating the configuration of pixels of the prior art liquid crystal panel. Shown in this figure are scanning signal lines 7 connected with the scanning-side driver IC 3, display signal lines 8 connected with the signal-side driver IC 2, switching elements 9 such as TFTs placed at the intersections of the scanning signal lines 7 and the display signal lines 8, and pixel electrodes 10 connected with the switching elements 9.
FIG. 12 is a cross-sectional view of the prior art liquid crystal panel, illustrating the cross-sectional structure of pixels. Shown in this figure are an array substrate 11 that is a first substrate, a counter substrate 12 that is a second substrate and located opposite to the array substrate 11, a common electrode 14 formed over the whole surface of the counter substrate 12, and a liquid crystal material 15 sealingly sandwiched between the array substrate 11 and the counter substrate 12. The first and second substrates 11 and 12 are made of glass. The pixel electrodes 10 are formed at individual pixels on the array substrate 11. The scanning signal lines 7, the display signal lines 8, and the switching elements 9 are also formed on the array substrate 11.
FIG. 13 is a diagram illustrating the waveforms of the display signal on each display electrode and of the potential on the common electrode of the prior art liquid crystal display. The display signal on the pixel electrode 10 is indicated by 6. The potential Vcom applied to the common electrode 14 is indicated by 16. The display signal 6 and the potential Vcom 16 shown in FIG. 13 are waveforms associated with one pixel. In FIG. 13, FO is an odd frame and FE is an even frame.
In the liquid crystal display of the construction as described above, the display signal is generally inverted in polarity every frame period at about 60 Hz to prevent the liquid crystal material from deteriorating due to aging. If the voltage applied to the liquid crystal material agrees with the center about which the polarity is inverted like the potential Vcom 16a shown in FIG. 13, the voltage applied to the liquid crystal material is constant with time. However, if the voltage deviates like potential Vcom 16b, and if the voltage value of an AC signal applied to the layer of the liquid-crystal material differs between when the polarity is positive and when it is negative, flicker (i.e., fluctuation of brightness) occurs at about 30 Hz.
To eliminate this flicker, it is necessary to adjust the level of the potential Vcom so that the voltage applied to the liquid crystal material does not differ between when the polarity is positive and when it is negative. In particular, an image producing easily discernible flicker is displayed on the viewing screen. Then, the operator adjusts the Vcom-adjusting knob mounted on the liquid crystal display, whereby the degree of flicker observed with the naked eye is minimized.
With this method, human factors vary the adjusted value of the potential Vcom. Accordingly, as shown in FIG. 14, another method uses an optical sensor in a certain location on the viewing screen. The resulting electrical signal waveform is observed. An adjustment is made to minimize the amplitude.
FIG. 14 is a block diagram showing the prior art image quality-adjusting system. The aforementioned optical sensor, indicated by numeral 17, is positioned opposite to a liquid crystal panel 1 and produces an electrical signal corresponding to the amount of light that the sensor receives. The output signal from the optical sensor 17 is amplified by an amplifier 18. A band-pass filter 19 is located on the output side of the amplifier 18 and detects a flicker signal component. The flicker signal from the band-pass filter 19 is indicated by 20. An oscilloscope 21 is used to observe the flicker signal 20. An image signal generator 22 produces an image display signal 23 to the liquid crystal panel 1.
A method disclosed in Japanese Patent Laid-Open No. 269991/1989 uses an optical sensor mounted opposite to a liquid crystal panel, a rectifier circuit for rectifying the output signal from the sensor that is in proportion to the light impinging on the sensor, and a low-pass filter for smoothing the rectifier output from the rectifier circuit and producing an output signal indicating the deviation from the optimum value of the potential on the common electrode. This method enables accurate adjustment.
In the above-described method using operator""s visual observation to make an adjustment for minimizing flicker, human factors vary the adjusted value of the potential Vcom. Especially, where the display screen is large, an optimum value of the potential Vcom at which flicker is minimized differs from location to location on the viewing screen. It is highly likely that the position at which an adjustment is made to minimize flicker varies, depending on the worker. As a result, fabricated products differ in performance.
Furthermore, the worker must watch flicker of high optical intensity for a long time. This may adversely affect the human body psychologically and physically.
In one of the above-described methods, the optical sensor located at some location on the viewing screen is used, the resulting electrical signal waveform is observed, and an adjustment is made to minimize the amplitude. In this method, the magnitude of the observed waveform differs according to the magnitude of the brightness of backlight and so it is difficult to detect the minimum value of the amplitude. Especially, immediately after the backlight is turned on, the brightness varies violently, thus deteriorating the efficiency of operation greatly.
Furthermore, the method disclosed in the above-cited Japanese Patent Laid-Open No. 269991/1989 makes use of the principle that the magnitude of a signal corresponding to the brightness is minimized. Therefore, the adjustment is directly affected by the brightness of backlight in the same way as in the above-described method.
In a further method, an adjustment is made with a frequency analyzer to minimize the frequency component corresponding to flicker. This method needs expensive apparatus. In addition, the response of the observed signal is slow. Hence, the efficiency of operation is poor.
The present invention has been made to solve the foregoing problems.
It is an object of the present invention to provide an image quality-adjusting system for a liquid crystal display, the system permitting one to adjust potential Vcom with high accuracy and high reproducibility for minimizing flicker.
An image quality-adjusting system for a liquid crystal display in accordance with the present invention comprises at least one optical sensor located opposite to a given location on a liquid crystal panel and an oscilloscope synchronized to a vertical synchronizing signal that is synchronized to odd frames or even frames. The optical sensor produces an output signal corresponding to the amount of light impinging on the sensor. The oscilloscope is used to observe the waveform of the electrical output signal from the optical sensor. A sufficiently high potential is previously applied to the common electrode of the liquid crystal display to observe a first waveform on the oscilloscope. Also, a sufficiently low potential is previously applied to the common electrode to observe a second waveform on the oscilloscope. The potential applied to the common electrode of a liquid crystal display to be adjusted is so adjusted that the waveform derived from this liquid crystal display and observed on the oscilloscope has a phase midway between the phase of the first waveform and the phase of the second waveform.
In one feature of the invention, the aforementioned at least one optical sensor consists of plural optical sensors. The resulting waveform of the electrical signals from the optical sensors is observed on the oscilloscope.
In another feature of the invention, the optical sensors are located on the same scanning signal line.
The plural optical sensors are positioned at least in a first measurement point and in a second measurement point. In the first measurement point, an adjusted value greater than a previously obtained, adjusted value of potential applied to the common electrode is derived. The previously obtained, adjusted value has been previously obtained by visual examination. In the second measurement point, an adjusted value smaller than a previously obtained, adjusted value of potential applied to the common electrode is derived.
In a further feature of the invention, at least one of the optical sensors detects the amount of received light via an optical attenuation filter.
In a yet other feature of the invention, an attenuation circuit is connected with the output of at least one of the optical sensors.
In an additional feature of the invention, the attenuation circuit has a variable attenuation factor.
The invention also provides a method of adjusting the image quality of a liquid crystal display, the method starting with setting the potential applied to the common electrode to a high value. Then, electrical signals from optical sensors located opposite to a given location on a liquid crystal panel are observed as a first waveform on an oscilloscope in synchronism with a vertical synchronizing signal that is synchronized to odd frames or even frames. The potential applied to the common electrode of the liquid crystal display is set to a sufficiently low value. The electrical signals from the optical sensors are observed as a second waveform on the oscilloscope. The optical sensors are placed opposite to a given position on the liquid crystal panel of a liquid crystal display to be adjusted. Then, the potential applied to the common electrode is adjusted such that the waveform of the electrical signals from the optical sensors observed on the oscilloscope has a phase midway between the phase of the first waveform and the phase of the second waveform, the optical sensors being located opposite to the liquid crystal display to be adjusted.
Other objects and features of the invention will appear in the course of the description thereof, which follows.